A helmet mounted or heads up visual display is often susceptible to illumination conditions where a viewer is incapable of seeing the display because the display brightness cannot produce sufficient intensity and contrast in order to display an image to the viewer. When a helmet mounted or heads up visual display is operating in very bright daylight, it is often possible for illumination conditions to exist wherein a pilot is incapable of seeing the display. Existing helmet mounted virtual displays cannot produce brightness of sufficient intensity to present rastered video imagery to a pilot above approximately 1.2:1 contrast ratio against through visor-seen brightness at or about 10,000 foot-lamberts. Video contrast requirements are in the 3:1 to 4:1 contrast ratio range. Higher brightness displays are possible but require active thermal cooling and add significant weight, volume and cost to the system. In the case of a pilot, the weight of added active thermal cooling causes the center of gravity to shift and introduce an undesirable top heavy center of gravity when the pilot is wearing the helmet. These reasons combine to make a helmet display utilizing a higher brightness unsuitable for use in high-g performance fighter aircraft. Therefore, a need is present to boost the display brightness in daylight that introduces no significant weight, volume, or power penalties to the helmet and display system.
In conditions with high levels of ambient light, it may become impossible for a viewer, (the pilot) to see the symbology contained within the display. This condition results when the backlit light emitting diode (LED) array fails to provide sufficient illumination required to provide contrast in high brightness environments. LEDs that are typically used to back light a liquid crystal display (LCD) display are approximately about three watts per display. In the bright environments these LEDs fail to provide a sufficient optical energy to create a usefully visible contrast within the display. Typically these conditions require a 2:1 or 3:1 contrast to properly see the display. One solution increases the power and brightness of the LED to maintain the required contrast in higher ambient illumination levels. However, heat dissipation problems arise as the output of the LED is increased as the LEDs are not 100 percent efficient. Typically, the currently available heat sinks are only capable of dissipating the heat associated with three watt LEDs. An increase above three watts requires additional cooling, which is typically supplied by liquid cooling.
Thus, the current technology fails to provide a display that can overcome high illumination environments and provide sufficient contrast in the display for the viewers. This failure results form inefficiencies within the LEDs and heat is created by available LEDs. Thus, the imagery will basically disappear above a certain light level.